Numpy 如何用另一个向量分配矩阵列的值？

Question

I'm trying to make an empty 3x2 matrix, then replace each column into randomly generated vectors.

Therefore, I tried to run the following code:

import numpy as np

A = np.empty(shape=(3,2))

x1 = np.random.rand(3,1)
x2 = np.random.rand(3,1)

A[:,1] = x1
A[:,2] = x2

However, when I tried to run the code, I got the following error message:

    A[:,1] = x1

ValueError: could not broadcast input array from shape (3,1) into shape (3)

How can I fix the error?

Thank you.

Answer 1

There are several things wrong here. First you're trying to assign to a slice a higher dimensional array:

A[:,0].shape
# (3,)

x1.shape
#(3, 1)

On another side, you're wrongly indexing, indices in numpy (and python more generally) begin at position 0 . So taking these aspects into consideration, you could either assign as:

A = np.empty(shape=(3,2))

x1 = np.random.rand(3,1)
x2 = np.random.rand(3,1)

A[:,0] = x1.ravel()
A[:,1] = x2.ravel()

A
array([[0.2331048 , 0.2974727 ],
       [0.6789782 , 0.9680256 ],
       [0.0151457 , 0.05476883]])

Or note that np.random.rand can generate arrays of multiple dimensions:

np.random.rand(3,2)
array([[0.10108146, 0.14859229],
       [0.55174044, 0.7399697 ],
       [0.38104021, 0.32287851]])
​

Answer 2

• A is of size 3 X 2 ie it has 3 rows and 2 columns
• A[:,1] implies all rows of A and second column. Array are 0 indexed in python
• A[:,1] is a column vector so you can assign any vector of size 3 into it
• np.random.rand(3,1) returns a numpy array (matrix) or size 3 X 1 . But what you want is a vector ie np.random.rand(3)

A = np.empty(shape=(3,2))

x1 = np.random.rand(3)
x2 = np.random.rand(3)

A[:,0] = x1
A[:,1] = x2